1. Field of the Invention
The invention relates to a process for the production of water-soluble or water-swellable polymers having a low content of residual monomers, particularly on the basis of acrylic acid and/or acrylamide and at least one additional water-soluble comonomer.
2. Description of the Background
Today, water-soluble or water-swellable copolymers are being used industrially in a variety of sectors. For example, they are used in a water-soluble form as flocculation aids in the clarification of municipal and industrial waste waters, as drilling fluids in petroleum production, as soil conditioners, thickening agents, dispersing aids, retention aids, etc.
Also, the fields of use of the corresponding crosslinked water-swellable polymers are highly diverse. Thus, for example, they are used in the personal care industry in the production of hygiene articles such as absorbent sanitary articles for infant and adult hygiene in the form of baby diapers, sanitary towels, tampons, incontinence articles, as well as products for wound dressing, in which they are incorporated.
These products utilize the characteristic absorbing properties of the water-swellable polymers because they are capable of absorbing a multiple of their weight of aqueous liquids or body fluids such as blood or urine with swelling and formation of hydrogels, and of retaining this volume of liquid even upon exposure to external pressure. These properties are utilized in the packaging industry in such a way that nonwoven products having the water-swellable polymers incorporated therein are used as packaging components, particularly in the form of absorbent inserts for fish and meat trays.
Other fields of use of such water-swellable polymers, usually being produced from an aqueous solution, forming a solid gel during polymerization, which is processed into a granular final product in the course of work-up, are agriculture and horticulture where, inter alia, water-swellable polymers are used as soil conditioners for water and nutrient storage, as artificial soil in plant breeding, and as root-protecting gels. In the cable industry and/or information technology, such water-swellable polymers are used as liquid-absorbing and/or liquid-sealing components in electric or optical fiber cables.
More specifically, the increasing use of acrylic acid-based crosslinked water-swellable polymers as so-called superabsorbers in hygiene products has resulted in intense activities of research and development, which are reflected by the range of related patent literature and also represent a direct consequence of the increased demands on the pattern of properties of these polymers made by the personal care industry.
While liquid-absorbing water-swellable polymers with very high swelling power or free swelling capacity upon contact with liquid initially had been used in diaper manufacture with preference, mechanical load studies on diapers led to the discovery that the strain caused by movements of a person wearing the diaper would require a different pattern of properties of the absorbing polymers used in the manufacture of diapers. To ensure liquid absorption, liquid transport, dryness of diaper and skin when using such superabsorbers in diaper constructions, a well-balanced relationship between the absorption capacity of the polymers (gel volume) and the gel strength of the hydrogel forming upon liquid absorption is required in association with high absorption capacity under pressure load.
To obtain superabsorbing polymers with the desired pattern of properties of high retention capacity, high gel strength and high absorption capacity under pressure, the surfaces of the absorbing polymers must be subjected to a subsequent treatment. Subsequent surface treatment has often been described in the patent literature:
Thus, U.S. Pat. No. 4,043,952 recommends polyvalent metal compounds to improve the dispersibility in water, and U.S. Pat. No. 4,051,086 recommends glyoxal to improve the absorption rate.
DE-OS 27 40 169 describes the production of absorbents based on polymers containing potassium acrylate and ammonium acrylate, which have been treated with polyols and used as powders and films in diapers and other hygiene products and in medical articles.
The patent documents EP 0 083 022, DE-OS 33 31 644, DE-OS 35 07 755, DE-OS 35 23 617, DE-OS 36 28 482, and EP 0 349 240 describe subsequent treatment of resins with crosslinking agents including two or more functional groups which may undergo reaction with the carboxyl or carboxylate groups or other groups contained in the polymer. Here, the powder is either mixed directly with the components, optionally co-using minor amounts of water and solvent, or the powder is dispersed in an inert solvent, or polymers containing 10 to 40 wt.-% water are dispersed in a hydrophilic or hydrophobic solvent and subsequently or simultaneously mixed with the crosslinking agent. Polyglycidyl ethers, haloepoxy compounds, polyols, polyamines or polyisocyanates can be used as crosslinking agents.
In addition to the above, DE-OS 33 14 019, DE-OS 37 37 196 also mention polyfunctional aziridine compounds, alkyl di(tri)halides and oil-soluble polyepoxy compounds.
In DE-OS 35 03 458, a crosslinking agent is applied on a polymer in the presence of an inert inorganic powder such as SiO2, without using organic solvents.
According to DE 40 20 780 C1, improved absorption under pressure is achieved by surface-crosslinking treatment of a polymer resin with 0.1 to 5 wt.-% of an alkylene carbonate.
All of the above-mentioned methods have in common that applying the surface-crosslinking agent is followed by thermal treatment of the polymers.
In view of the primary use of such crosslinked absorber polymers in the hygiene sector, special care should be taken that the polymers intended for use in baby diapers or other hygiene articles would not cause any skin-sensitizing effects in the user of such products. More specifically, it must be ensured that the starting materials and crosslinking agents used in the production process are reacted in such a way that any potential of hazard to health of the final product can be excluded, because the final product is to be rated as safe in toxicological terms, i.e., the polymers must have zero or only a very low content of residual monomers whose toxicity has been well researched by experiments.
However, this not only applies to polymers intended for use in hygiene products, but also to water-soluble and water-swellable polymers based on acrylamide. Thus, the toxic potential of acrylamide has been known for a long time, and a large number of patents and publications are dealing with the reduction of acrylamide in such polymer products. One way of achieving a reduction of the residual monomer content of acrylamide is based on the addition of an additive following polymerization, which adds to the double bond of the acrylamide residual monomer, thereby reacting same.
It has been found, however, that complete conversion of the monomers, especially monomers based on acrylic acid and those based on acrylamide, is not possible in the production of water-soluble or water-swellable polymers. Thus, it has been determined that the production of a crosslinked water-swellable polymer having the desired properties of high absorption rate, high retention capacity and high absorption under pressure load, in association with a residual monomer content as low as possible, depends—among other things—on the crosslinker concentration and the polymerization catalysts being used.
High retention capacity is achieved in a polymer having a low level of crosslinking. In contrast, high absorption under pressure requires a highly crosslinked and thus stabilized polymer which is capable of swelling against external pressure only as a result of its stability. As crosslinking increases, however, the residual monomer content likewise increases, because the rapidly increasing viscosity during production of the polymer reduces diffusion of the residual monomer molecules to the reactive free-radical centers. Moreover, catalyst systems reducing the level of residual monomers or high-level catalyst systems frequently result in an undesirable reduction of retention.
Due to the above-mentioned problems in conducting a polymerization process in such a way that virtually no residual monomers would be formed, and as a result of the toxicological safety demanded of the final product, various methods have been proposed in which the residual monomers are removed from the previously produced polymers by converting the former into harmless derivatives.
For example, DE-AS 10 70 377 and U.S. Pat. No. 2,960,486 describe treatment of the high-molecular weight polymer gel, which is obtained following polymerization, with aqueous sodium disulfite or sodium metabisulfite solution and drying thereof at 80 to 120° C., thus obtaining a residual acrylamide content of 0.01%. Due to their economic inefficiency, these methods failed to achieve general acceptance in practice, because they require processing of the polymer products in a highly diluted (2 to 3%) polymer solution.
Likewise, JP-PS 56/103207 uses bisulfites, sulfites and pyrosulfites to reduce the level of residual monomers. U.S. Pat. No. 3,780,006 uses gaseous sulfur dioxide in order to decrease the acrylamide concentration in an emulsion polymer.
The treatment of a polymer gel with an aqueous solution of sodium bisulfite or metabisulfite has been described in U.S. Pat. No. 3,755,280, and with a solid alkali sulfite in EP 175,554, where residual monomer contents of from 0.03 to 0.3 wt.-% have been obtained.
EP 0 505 163 describes a secondary treatment of crosslinked acrylic acid polymers with a combination of metabisulfite and a surface-active substance (HLB from 3 to 40), thereby decreasing the residual monomer content down to 10 ppm. From 2 to 5 wt.-% of metabisulfite (relative to the polymer gel having 40% w.s., that is, from 5 to 12.5 wt.-% of metabisulfite relative to the dry polymer product) is necessary in such a secondary treatment of polymers in order to achieve the above-mentioned decrease in residual monomers. Such high quantities of added substances may have adverse effects on the technological properties.
WO 94/20547 describes additives such as bromates and chlorates in the polymerization solution and subsequent heating of the final polymer wherein, inter alia, a decrease of the residual monomers is effected by the additives. The bromates and chlorates may also be added subsequent to the polymerization. Despite these measures, the residual monomer content of the polymer products is between about 135 and 1100 ppm.
EP 0 303 518 A2 describes a continuous and discontinuous process, respectively, for the production of crosslinked polyacrylate resins, wherein acrylic acid with a degree of neutralization of 70 to 100 mole-% and a water-miscible or water-soluble polyvinyl monomer are polymerized in aqueous solution. The aqueous monomer solution to be polymerized has a monomer concentration of at least 50%, and a combination of a thermal polymerization initiator and a redox initiator is employed to reduce the level of monomers. The polymerization is conducted in such a way that the exothermic heat of reaction generated during polymerization is essentially the only thermal energy that is used to perform polymerization, crosslinking and evaporation of the water employed, so that subsequent drying can be omitted. In general, the polymer products obtained in this way are said to have residual monomer contents below 500 ppm, preferably below 200 ppm.
While the above-mentioned processes allow production of polymers with reduced residual monomer content, they frequently require complicated reaction engineering, considerably raising the production cost of the polymer products. In particular, secondary treatment with sulfur-containing compounds, part of which remain in the final product as additives, must be regarded as disadvantageous. Such secondary treatment—as an additional reaction step—not only involves the use of additional apparatus or equipment etc., i.e., is associated with an additional input of time and, as a consequence, cost, but also, it does not exclude formation of oxidation products of sulfur which must be taken into account not only from an environmental point of view, but also, in particular, technologically with respect to corrosion resistance of the plant.
Furthermore, treatment with cysteine is not possible for water-soluble copolymers, because this would give rise to crosslinking and, as a consequence, insolubility of the product. Moreover, the high amounts of sulfite, in some cases 2 to 3% relative to the weight of the product, have an extremely negative effect on the technological properties of the product.
DE 35 39 385 therefore claims treatment of acrylamide polymers and copolymers with amines.
DE 19 752 127 and DE 19 752 128 describe addition of ammonia and amines or ammonium salts to the monomer solution. This process is disadvantageous in that the product must be dried above 120° C. in order to obtain the desired effect. This results in a significantly deteriorated solubility for water-soluble polymers as a result of crosslinking reactions taking place at elevated temperature. In particular, this applies to copolymers including non-ionic comonomers such as sodium acrylate in addition to acrylamide.
WO 01/25289 describes irradiation of the polymer gel following crushing. Therein, polymerization is effected in the absence of light and in the presence of a UV initiator which is activated by UV light following crushing. The same procedure has been described in WO 01/55228.
These methods involve the following drawbacks:    a) An additional additive (photoinitiator) not utilized in the actual polymerization must be added to the monomer solution.    b) The actual polymerization must be carried out in the absence of light, which is complicated in terms of production technology.    c) Due to its short wavelength, UV light has only a short penetration depth into the polymer following crushing.
The object of the present invention was to devise a process that would furnish polymers, especially on the basis of acrylic acid or acrylamide, which contain a very low or zero residual content of residual monomers, in which process the amount of unreacted monomers is to be significantly reduced or brought to complete conversion even during polymerization, so that operation may proceed using less residual monomer scavenger or none at all. Furthermore, the process should work without additives having no function during polymerization, or, in other words, it should be possible to perform the process in a continuous fashion on a continuous belt.